1. Field of the Invention
The present invention is directed to the degassing of liquid monomer used in fabricating ophthalmic lenses, such as for example soft contact lenses. In particular, the invention concerns a packing element for a degasser. The packing element is modular, and adapted for stacking with other such modules. The invention provides increased flexibility for making modifications to the degas operation by allowing convenient addition, removal or replacement of such packing elements as may be required in response to any given situation. Moreover, the packing element of the invention can be more easily cleaned, which operation is further facilitated by the packing element being constituted of removable parts. The invention also permits in-line degassing of liquid monomer, which increases ophthalmic lens production line efficiencies and mitigates environmental concerns associated with batch degassing processes commonly used.
2. Description of the Prior Art
Ophthalmic lenses, such as for example, soft contact lenses, can be fabricated by a variety of techniques. The more industrially popular methods involve the use of suitable liquid monomers, such as hydroxyethyl methacrylate (HEMA), that are deposited into especially configured mold halves. The monomer can be cured by any number of techniques, e.g. ultraviolet radiation, to form the lens. The liquid monomer, however, invariably contains dissolved gases such as oxygen and nitrogen, which can detrimentally affect curing either by forming unwanted bubbles, which can manifest as voids or other flaws in the final lens, or by otherwise interfering with the curing mechanism itself. It has thus become a common practice to degas the liquid monomer prior to use.
Accordingly, different techniques have been explored for degassing the monomer. One practice is to employ a rotary evaporator unit that removes excess gas from the monomer by rotating same under subatmospheric pressure. The container holding the mixture is then flushed with and held under nitrogen atmosphere. Another technique is described in U.S. Pat. No. 5,435,943 wherein the monomer is pumped through a gas permeable tube surrounded by a subatmospheric chamber. Gases in the monomer permeate the tube in favor of the lower pressure on the outside of same, the degassed monomer then being deposited into the lens molds and cured.
While these methods have proven commercially useful, efforts to advance the degas operation are nonetheless of interest. For example, the rotary evaporator method provides an opportunity for nitrogen gas to re-dissolve into the monomer during back fill flushing. Degas using permeable tubing has its own drawbacks: it typically demands batch operation due to the particulars involved in pumping viscous liquid monomer through tubing; thus, monomer is stored in vessels until required, whereupon it is sent to a tubular degas station which is off line. In a production environment where automation and advances in automation are critical, the use of a batch operation causes numerous inefficiencies that can adversely affect yield and logistics. Related to this is the fact that the permeable tubing is at some point subject to breakage, due for example to the pressure difference on either side of the tube wall and wear of the material of construction, typically silicone tubing. Breakage usually results in a shut down and often requires the wholesale replacement of tubing bundles, even those that are still intact, the arduousness of which can further disrupt operations. Moreover, the permeable tubing typically can not be properly cleaned when the need arises given, for example, the difficulty of cleaning the lumen. A batch degas operation also creates disposal problems inasmuch as liquid monomers for ophthalmic lenses commonly have shelf lives, and if not used within same they must be discarded. Moreover, in a batch operation where liquid monomer is pumped from a storage vessel, there is always some residual monomer remaining in the vessel after use. The aggregate amount of this residue, in the context of an industrial production facility, must be disposed of once past its shelf life.
There is thus a continuing need in the art for a degas technology that has increased effectiveness in both operation and maintenance, which technology can be employed in-line, and with reduced disposal issues.
The present invention satisfies the foregoing desiderata. The invention is directed to a stackable packing element for a degasser employed to degas liquid monomer used to fabricate ophthalmic lenses. The stackable packing element comprises:
a body module having a bottom surface with at least one hole therethrough and an upwardly directed sidewall peripheral to said bottom surface and adapted for stacking with another body module, said bottom surface and said upwardly directed sidewall defining a chamber; and
a puck having a top portion onto which said liquid monomer can flow, and a side member, said side member extending downwardly from said top portion to removably set said puck within said chamber, said side member defining at least one side opening through which said liquid monomer can flow from said top portion into said chamber and over said bottom surface and through said at least one hole.
The invention also relates to a degasser comprised of a plurality of said stackable packing elements, as well as a degassing process employing same. The degasser can comprise modular packing elements.
Beneficially, the invention enables thin film flow of liquid monomer over the surfaces defined which allows direct contact of the monomer with the vacuum environment or an inert environment, preferably in a vacuum environment, under which the degasser operates, thus providing more efficient degas than heretofore obtainable. The invention can also be implemented in-line with the production facility, thus eliminating the difficulties inherent to batch degas processing as set forth hereinabove. That is, using the invention, degassing can occur continuously with the degassed monomer being pumped directly to the deposition station on the production line. Furthermore, by being modular in nature, the invention enables quick reconfiguration of the degasser by simply stacking more packing elements to, or conversely by simply removing same from the stack. Additionally, the modularity of the packing elements and their surfaces are more easily cleaned than prior art devices; this is further facilitated by the packing element being constituted of removable parts.
In an alternative embodiment a plurality of stacking packing elements are assembled within a container, such as a column, but not attached to the container. Preferably, first and second packing elements alternate within the container. The packing elements have vertical supports between horizontally extended pieces. The flow of the liquid is from the uppermost packing element to the lowermost packing element. The plurality of stackable packing elements comprise at least two differently shaped modular packing elements that alternate in the stack. In the preferred embodiment, the flow of the liquid within the container is from the center of a first packing element towards the periphery of the first packing element then to the periphery of a second packing element located beneath the first packing element. The liquid then flows from the periphery to the center of the second packing element under vacuum or an inert environment within the container. In this embodiment the stackable packing elements do not themselves define the chambers within which a vacuum is established or through which an inert gas is pumped.